The present disclosure relates to processes for cleaning mixing devices that are used to make thermoplastic polymers and polymer blends. The processes aim to increase the speed of the cleaning process, provide lower resin degradation, and allow for a higher overall extrusion rate. The disclosure also relates to purge compositions useful in such processes.
Extruders using a melt conveying screw accomplish a large portion of the melt processing of thermoplastic resins. The extruder can be used for compounding, molding, pelletization or forming films, sheets or profiles. Such extruders typically have a heated extrusion barrel and one or two screws revolving within the barrel to compress, melt, and extrude the resin through an orifice or die. The barrel is divided into several different zones, such as feed, transition, mixing, dispersion, and metering zones. When such machines are dedicated to making a single material in a single color, they can be very efficient. However, when the same machine is used to make a variety of materials and/or colors, there is a need to changeover or switch between materials (i.e. changeover process). Otherwise in this situation, impurities or residues created during the extrusion of the first, preceding resin can contaminate the second, succeeding resin. During this changeover process, the extruder produces a combination of the preceding material/color and the succeeding material/color. As a result, the combination may not meet desired specifications, e.g. compositional differences, a loss of clarity, a change in color or viscosity, or some other defect. In addition, the changeover process leads to a loss of material and a decrease in manufacturing efficiency.
These deficiencies have been addressed to some extent in several ways. For instance, a high viscosity or sticky resin has been used to push out the old material. While this can be effective, the high viscosity resin itself must be pushed out of the machine, leading to inefficiency. In other instances, fiber glass reinforced resins are used as a purge compound to clean out the extruder. While effective in some systems, this process again leaves the problem of removing the fiber glass resin itself.
In other instances resins such as polymethylmethacrylate based purge compounds (commercially sold as RIDD) are used to purge melt processing equipment at high temperatures (e.g. over 250 degrees Celsius (° C.)). These resins give off objectionable odoriferous fumes that cause irritation to the operators and those around them, and are increasingly unacceptable in a modern manufacturing environment.
Thus, there exists a need for processes and compositions to quickly changeover a screw-based melt processing machine from one polymeric material to another polymeric material in a short period of time, with no degradation of the polymer and with no fumes due to their use. Desirably, the process/composition should be applicable for cleaning a wide variety of contaminants, especially colorants such as carbon black, titanium dioxide, anthraquinone and phthalocyanine-based pigments and dyes as well as removing black specks, crosslinked resins, and gels caused by degradation of resins or additive during melt compounding and melt processing.